The present invention relates to an exposure system for fine semiconductor integral circuit patterns and, more particularly, to a vertical XY stage which can be suitably applied to an exposure system using synchrotron orbital radiation as a light source.
In the process of manufacturing a semiconductor device, an exposure system is used to expose circuit patterns on a semiconductor device substrate called a wafer coated with a sensitive material. In order to expose identical circuit patterns at a large number of portions on the wafer, the exposure system uses a vertical XY stage for moving the wafer in two axial directions.
With an increase in degree of integration of a semiconductor device, the wavelength of a replicating light source is reduced. As a future technique, a pattern replicating technique using synchrotron radiation (to be referred to as SR light hereinafter) has been developed. The wavelength of SR light is 1/100 or less that of an ultraviolet ray as a photo-lithography light source. Such short-wavelength light is used because diffraction and interference are reduced to allow replication of fine patterns as the wavelength of light is shortened.
This SR light is defined as a beam of an electromagnetic wave which is radiated in the tangential direction when a high-energy electron beam propagating at a speed approaching the velocity of light is bent by a magnet. The SR light is highly directional high-brightness light including light components from visible light to an x-ray. Of these light components, a soft x-ray having a wavelength of about 1 nm is extracted to be used as a light source for a pattern replication.
FIG. 8 shows a conventional vertical XY stage used in an exposure system using such SR light as a light source. This vertical XY stage will be briefly described below. Referring to FIG. 8, reference numeral 1 denotes SR light, which is radiated in all directions within a plane parallel to tangents along which electron beams propagate from a doughnut-like ring 2 as a closed orbit within which high-energy electron beams are stored. An x-ray mask 3 is arranged in one of the directions of the SR light 1. A circuit pattern is formed on the x-ray mask 3. The SR light 1 is transmitted through the circuit pattern on the mask to form the shadow of the pattern onto a wafer. Reference numeral 4 denotes a mask stage for holding the x-ray mask 3 and controlling its position and posture; 5, a wafer stage for holding a wafer 6 by suction; 7, a detection system for observing alignment marks formed beforehand on the x-ray mask 3 and the wafer 6 to optically detect positioning errors; and 8, an XY stage for moving the wafer 6 in the X and Y directions. After positioning of the XY stage 8 and the mask stage 4 is performed by moving/adjusting them in accordance with positioning error detection signals based on the alignment marks, the SR light 1 is radiated to replicate the pattern of the x-ray mask 3 onto a sensitive material coated on the wafer 6.
The SR light 1 is normally radiated within a horizontal plane. For this reason, the x-ray mask 3 and the wafer 6 are vertically arranged within a vertical plane, and the XY stage 8 is of a vertical type. Although the SR light 1 is highly directional, it has a slight divergent property. If, therefore, the distance between the x-ray mask 3 and the wafer 6 is increased, a projected pattern shape is distorted or blurred. This makes it difficult to perform normal exposure. In order to prevent this, the distance between the x-ray mask 3 and the wafer 6 must be set to be very small, e.g., 50 .mu.m or less.
Referring to FIG. 8, the wafer 6 is not on the SR optical axis where the x-ray mask 3 is set and is located at the position far from the mask 3, so the x-ray mask 3 and the wafer 6 are easily exchanged with other ones. When only an exposure process is taken into consideration, it is only required that the XY stage 8 be moved by an amount corresponding to an exposure area of the wafer 6. However, the x-ray mask 3 and the wafer 6 need to be sequentially exchanged with other ones. If, therefore, the XY stage has only a moving amount corresponding to the exposure area, it is almost impossible to exchange the x-ray mask 3 and the wafer 6 without damaging them in a state wherein they face each other with a small clearance secured therebetween. For this reason, after the x-ray mask 3 and the wafer 6 are exchanged with other ones in directions indicated by arrows 9 and 10 in FIG. 8, the wafer 6 is moved, by the XY stage 8, to a position where it faces the x-ray mask 3, and an exposure process is started.
The XY stage 8 comprises an X-axis guide 11, an X-axis slider 12, and an X-axis lead screw 13, which serve as a guide unit for moving the wafer 6 in the X direction, i.e., in the horizontal direction, and a Y-axis guide 14, a Y-axis slider 15, and a Y-axis lead screw 16, which serve as a guide unit for moving the wafer 6 in the Y direction, i.e., the vertical direction. In general, the Y-axis slider 15 is mounted on the X-axis slider 12, and the X- and Y-axis guides 11 and 14 serve as stationary members so that the X- and Y-axis sliders 12 and 15 are respectively moved in the X- and Y-axis directions by the lead screws 13 and 16. Reference numeral 17 denotes an angle plate for vertically connecting the Y-axis slider 15 onto the X-axis slider 12; 18, an X-axis driving motor; and 19, a Y-axis driving motor. In this case, the load of the X-axis driving motor 18 is larger than that of the Y-axis driving motor 19 because the motor 18 is designed to drive the X-axis slider 12, the angle plate 17, the Y-axis guide 14, and the Y-axis slider 15 together. The X-axis driving motor 18 inevitably has a larger capacity. The X-axis slider 12 is particularly required to have a moving amount equivalent to the sum of a stroke corresponding to the exposure area and a stroke required for exchange of the x-ray mask 3 and the wafer 6. That is, the X-axis slider 12 is required to have a moving amount about three to five times the exposure area. Consequently, the X-axis slider 12 is longer than the Y-axis slider 15, resulting in an increase in width of the vertical XY stage 8 in the horizontal direction.
As described above, in the conventional vertical XY stage 8 used in an SR exposure system, since the X-axis slider 12 is required to have a moving amount about three to five times the exposure area, the width of the stage 8 in the horizontal direction is inevitably increased, resulting in the following problems.
1 As described above, the SR light 1 is radiated in all the directions within the plane parallel to the tangents along which electron beams rotate. Therefore, as the width of the vertical XY stage 8 is reduced, a larger number of exposure systems can be arranged in the radial direction around the ring 2, allowing the effective use of the ring 2. In the arrangement of the conventional vertical XY stage 8, however, in order to facilitate exchange of the x-ray mask 3 and the wafer 6, the width of the XY stage 8 must be increased in the horizontal plane direction perpendicular to the axis of the SR light 1, i.e., the X-axis direction. That is, the arrangement of the conventional vertical XY stage 8 is against the effective use of the ring 2.
2 The lead accuracy of the XY stage 8 deteriorates. It may be considered that the lead accuracy of the XY stage 8 is determined by guide accuracy and lead accuracy in holding the posture of the stage, which are determined by machining accuracy of constituent members. In some method, a mechanism which slightly moves by an amount corresponding to the lead error of the XY stage 8 and an error measurement system are independently added to correct the error, thus increasing the accuracy. In this method, however, a mechanical system, a measurement system, a control system, and the like, each having accuracy or performance 10 or more times higher than that of the main body must be additionally mounted, resulting in an increase in weight and cost. Besides, the stage is complicated and easy to fail. In order to prevent such inconvenience, it is preferable that the final accuracy be ensured by only the main body. Therefore, it is important for the XY stage to have the simplest structure and a shape allowing an increase in machining accuracy of constituent members.
If the length of the X-axis slider 12 is increased, the X-axis guide 11 and the X-axis lead screw 13 must be increased in length accordingly. The machining accuracy is inversely proportional to the length even if the member strength is infinite. In addition, the member strength is inversely proportional to the third power of length and the fourth power of diameter. More specifically, in machining of a member, various types of force and stress, e.g., cutting force and grating force, residual stress and thermal stress accompanying plastic deformation, are applied to the member. In addition, when the member is fixed to a machining unit, chucking force and the like act on the member. If such force acts on the member, the member is deformed. As the member is deformed, machining accuracy deteriorates. That is, the strength of a member is the largest factor which affects the machining accuracy.
In other words, an increase in length of a member greatly decreases the machining accuracy, and this decrease in machining accuracy causes a deterioration in lead accuracy of the XY stage.
1 The Y-axis slider 15 is unbalanced in weight. The weight applied to the X-axis slider 12 perpendicularly acts on the X-axis guide 11 and hence is supported by the guide surfaces. In this case, the static rotational load applied to the X-axis lead screw is only a friction force on the tooth surfaces, but the weight of the X-axis slider 12 itself does not act as a load. In contrast to this, the weight applied to the Y-axis slider 15 acts in the guiding direction of the Y-axis slider 15 and hence is supported by the Y-axis lead screw 16. Therefore, the weight of the Y-axis slider 15 directly acts as a rotational load on the Y-axis lead screw 16. This rotational load based on gravity acts in a direction opposite to the rotational direction of the Y-axis driving motor 19. This means that energy supplied to the Y-axis driving motor 16 must be changed depending on whether the Y-axis slider 15 is moved upward or downward. Such a change in energy adversely affects driving control of the Y-axis slider 15. As a result, the positioning accuracy of the Y-axis slider 15 deteriorates, or an increase in feed speed is hindered.
Note that the weights of the X- and Y-axis sliders 12 and 15 are preferably minimized. This is because a decrease in weight of each slider reduces the load applied to the driving motor and allows control to perform high-speed, high-accuracy positioning.
4 The conventional XY stage is lacking in safety against obstacles. The X-axis slider 12 is reciprocated between the exchange position for the wafer 6 and the exposure position. No problem is posed in forward movement. In backward movement, however, since the mask stage 4 and the wafer stage 5 pass each other, if the hand or finger of an operator is caught therebetween at the time of starting/maintenance of the apparatus, the operator is might be seriously injured. This is because the weight of the X-axis slider 12 itself and the weights of the Y-axis guide 14, the Y-axis slider 15, and the angle plate 17, which are supported by the X-axis slider 12, amount to a considerable weight, and such a heavy part does not easily stop once it is moved. If the motor is being driven, it is still more difficult to stop the X-axis slider 12. Under the circumstances, safety measures demand an excessively large load.